About Graphical Severe Thunderstorm Warnings in the ACT
The Bureau of Meteorology is now providing enhanced, graphical, severe thunderstorm warning services for NSW and the ACT. The aim is to better define the regions under threat when dangerous thunderstorms are occurring. The new products contain graphics that will complement the Bureau's traditional text-based warning products.
Severe ThunderstormsA severe thunderstorm is a thunderstorm that produces one or more of the following:
For more information about Severe Thunderstorms refer to the Severe Thunderstorms Brochure
Only about 10 percent of thunderstorms are severe, but these account for approximately 90 percent of the damage produced by all thunderstorms.
Overview of the Severe Thunderstorm Warning Service in NSW and the ACT
The Bureau of Meteorology issues Severe Thunderstorm Warnings whenever severe thunderstorms are occurring in an area or are expected to develop or move into the area during the ensuing few hours. The warnings describe the area under threat and the particular hazards likely to be associated with the thunderstorms. These warnings are distributed widely to the media and emergency services, and are available to the public via the Web and various telephone and fax-based services. An image is available with the warning on the Web to show, in map format, the area at risk. This service is provided for all parts of NSW and the ACT.
When severe thunderstorms are actually occurring, or are about to move into, the heavily populated region around Sydney, Newcastle and Wollongong, or are affecting or about to affect Canberra or Queanbeyan, the Bureau of Meteorology may issue a more detailed Severe Thunderstorm Warning for people in the affected region. High quality, full-time weather radar coverage in these areas allows the warnings to describe the current location of individual thunderstorms, and the places likely to be affected within the next 30 to 60 minutes. Again, an image is available with the warning, in this case showing in map format the location of the severe thunderstorms and their forecast tracks.
Severe Thunderstorm Warnings for NSW and the ACTBasis of Warnings
These warnings are issued as an alert to the public, emergency services and other organisations when severe thunderstorms are likely to develop, or extend into, a specified area over the next few hours. The warnings are issued, as necessary, for all parts of NSW and the ACT. A sample warning can be found below.
Important things to note
The area covered by the Warning and under threat from severe thunderstorms is indicated by yellow shading. The area may consist of all or part of one or several weather forecast districts.
The issue time is the local time at which the Bureau transmitted the Warning.
Warnings for NSW and the ACT are valid for up to 6 hours but are updated as a weather situation evolves, usually every 3 hours, but more frequently if the situation warrants. The expected issue time of the next Warning is given in the text.
Districts shown on map
The divisions on the chart indicate the boundaries of the Bureau’s weather forecast districts. Each district is labelled with an abbreviation, as follows:
Individual thunderstorms are small-scale and short-lived phenomena - a thunderstorm is typically only about 10 km across and lives for only 30 minutes or so. At any one time there can be many thunderstorms affecting a district, only some of which may be severe. It is important to realise that not all locations highlighted in a Warning will experience severe thunderstorms. The Warning only indicates that some of the thunderstorms in the area are expected to be severe.
Severe Thunderstorm Warning - Canberra
Basis of Warnings
The Severe Thunderstorm Warning - Canberra is a more detailed alert issued to the public, emergency services and other organisations when severe thunderstorms are actually detected in the Canberra region and are expected to affect Canberra and suburbs or Queanbeyan within the next 60 minutes.
These Warnings depict and describe individual severe thunderstorms and therefore rely heavily on a detailed analysis of radar data. The Bureau’s radar display systems can visualize thunderstorms in 3-dimensions (as opposed to the 2-dimensional display available on the internet), allowing meteorologists to search for and identify severe features that are not apparent at ground level.
Meteorologists also rely on frequently updated data from automatic weather stations, measurements of upper air conditions from weather balloons, atmospheric profilers and specially instrumented aircraft using Canberra Airport, as well as computer-generated analysis and forecast charts.
A sample warning is shown below.
Important things to note
The location of each severe thunderstorm is indicated by a red ellipse. The ellipses show the positions of the thunderstorms at the "valid time" stamped on the image. This will generally be a few minutes before the Warning was issued. The thunderstorm positions shown are derived from radar data. The graphic shows a simplified representation of the situation, which may actually be very complex.
Note particularly that the map will only depict severe thunderstorms likely to affect Canberra and suburbs or Queanbeyan (or thunderstorms imminently expected to become severe, which are expected to affect these areas). Non-severe thunderstorms are not shown, and severe thunderstorms that are not forecast to affect Canberra and Queanbeyan will not usually be shown. People outside of Canberra and Queanbeyan should not use this product but should instead refer to the general NSW/ACT Severe Thunderstorm Warning product available under the Warnings heading at http://www.bom.gov.au/act/ and http://www.bom.gov.au/nsw/.
An arrow indicates the forecast direction of movement of each severe thunderstorm. This is the direction towards which the thunderstorm is moving. Arcs are used to show the forecast positions of the front edge of the thunderstorm at 10 minute intervals.
The number of ten-minute forecast positions shown will depend on the current behaviour of the thunderstorms. Usually six arcs will be shown, giving forecast positions for the front edge of the thunderstorm out to 60 minutes from the valid time. Sometimes, for longer-lived thunderstorms, it may be possible to extended forecast positions out to 90 minutes. On other occasions, individual severe thunderstorms may be expected to last only for a short period and tracks will be shown only for the next 30 minutes.
The yellow “Warning Area” shown on the map indicates the general area considered under threat from severe thunderstorm activity over the next several hours. Because of this longer time-frame, it will usually depict a more extensive area than that currently affected by severe thunderstorms. This yellow "Warning Area" is derived from the broad-scale NSW/ACT Severe Thunderstorm Warning that will normally be in effect to describe the severe thunderstorm threat over the entire NSW/ACT region.
The orange cross-hatched "Immediate Threat Area" shows the parts of Canberra and Queanbeyan forecast to be at risk from severe thunderstorms during the validity of the present Severe Thunderstorm Warning - Canberra - usually the next 60 minutes. It too may be larger than the area under the forecast tracks of the severe thunderstorms shown on the graphic. This is because forecasters need to consider the possibility that thunderstorms might deviate from the forecast tracks, and must allow for the development of new severe thunderstorms or the intensification of thunderstorms not yet meeting severe thunderstorm criteria.
Issue time and Valid time
The issue time is the local time at which the Warning was transmitted by the Bureau of Meteorology. The valid time is the time of validity of the initial severe thunderstorm locations shown by red ellipses on the chart.
Warnings are generally valid for 60 minutes (rarely, up to 90 minutes) and will be updated every 30 to 60 minutes, or as required, as a weather situation evolves.Area covered
These more detailed warnings are issued for people in Canberra and suburbs and Queanbeyan.
Major localities in the area include
The following suburbs and localities are covered
The warning area is based on the Canberra/Queanbeyan urban centre (as defined by the Bureau of Statistics), and the suburbs and divisions used by the ACT Planning and Land Authority.
The area is outlined by the thin line on the map that accompanies the Severe Thunderstorm Warning - Canberra product.Other areas
This warning is intended for people in the Canberra (and suburbs) and Queanbeyan only. It will only show severe thunderstorms if they are expected to affect Canberra and suburbs or Queanbeyan in the next hour. Severe thunderstorms affecting other areas will not be shown. People in surrounding areas should not use this product but should refer to the general NSW/ACT Severe Thunderstorm Warning product available under the Warnings heading at http://www.bom.gov.au/act/ and http://www.bom.gov.au/nsw/ .Non-severe thunderstorms
Only thunderstorms that are identified as severe, or expected to become severe (according to the definition above), are depicted and described in a Warning. Other thunderstorms may be occurring but these will not be indicated on the map.
Warnings titled NSW/ACT Severe Thunderstorm Warning and Severe Thunderstorm Warning - Canberra are relevant to people in Canberra and Queanbeyan, with the latter warning providing greater detail when forecasters are able to provide it.
People in surrounding areas should refer to the NSW/ACT Severe Thunderstorm Warning product.